1. Field of the Invention
This invention relates broadly to industrial process control instrumentation. More particularly, this invention relates to improved electrical display stations of the type which are connected to process control instrumentation for the purpose of providing operating personnel with a visual display of the amplitude and status of signals pertinent to the process being controlled and providing means which enable the operating personnel to exercise control over the process.
2. Description of the Prior Art
Electrical display stations have been in use in the process industry for many years. During these years, the amount of information required by operating personnel in order to exercise control over a process has increased. Much of the information has been available but at various locations where it is not directly viewable by one person. For example, the electrical display stations have provided one person with the indication of the measurement and set-point signals. However, other signals, such as ratio and alarm signals have been available only at remote locations. This situation has lead to the need to have one person make an adjustment at the remote location while another person observed the result of the adjustment at the electrical display station and relayed instructions back to the person making the adjustment.
In attempts to provide the type of information required at the electrical display station, manufacturers have been faced with the problem of supplying a greater selection of embodiment of stations to meet the requirements for each process and each industry. However, these attempts have still not solved the problem of consolidating all of the information and adjustments at one station.
Until recently, electrical display stations have used instruments having moving pointers, such as ammeters or voltmeters, to display the amplitude of signals. As is well known, instruments having moving pointers are subject to changes in calibration due to vibration and wear. As a result, it has been necessary to calibrate each station at the factory and recalibrate it after installation. Thereafter, a station required recalibration at intervals depending on the conditions to which the station was subjected.
Recently electrical type of display panels, such as light emitting diodes and gaseous discharges devices, have come into use to display signals in bar graph mode. Inasmuch as these types of displays have no moving parts, once they are calibrated at the factory, they retain the calibration. Therefore, they eliminate the need for recalibration. However, to date, the electrical display stations utilizing these electrical type of display panels have merely followed past practices insofar as the signals displayed and adjustments available are concerned. In addition they have used conventional circuits for controlling the energization of the display panels. These conventional circuits have included oscillators and clock circuits for controlling the scanning circuits. If any other functions were performed by a station, such as computations, independent conventional circuits have been used in addition to the conventional circuits for controlling the energization of the display.
Also quite recently, cathode-ray tubes have been employed to display selected signals. The cathode-ray tubes have been limited to use in connection with digital process control systems. Due to the relatively high cost of digital control systems, they are not economically feasible unless a large number of process control loops are involved. This precludes the use of cathode-ray tubes for electrical display stations for users having too few process control loops. Further, regardless of the number of available process control loops, many users prefer the simplicity of an electrical display station dedicated to a single process control loop.
In the past, because of the construction of the electrical display station, it has been necessary to use trained personnel to assemble a station. In view of the wide variety of embodiments of electrical display stations required to provide a complete line to the process industries, manufacturers have had to carry an inventory of each embodiment or assemble each embodiment upon receipt of an order. In the event a customer needed to have an electrical display station modified, it had to be returned to the manufacturer for modification or exchanged for another embodiment.